Groundwater Discharge by Evapotranspiration, Flow of Water in Unsaturated Soil, and Stable Isotope Water Sourcing in Areas of Sparse Vegetation, Amargosa Desert, Nye County, Nevada
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The USGS conducted a study to evaluate the potential for groundwater discharge from sparsely vegetated areas in the Amargosa Desert. The study objectives were to: (1) compute groundwater discharge based on evapotranspiration and precipitation measurements at instrumented sites, and (2) improve understanding of hydrologic-continuum processes controlling groundwater discharge through analysis of complementary saturated zone, unsaturated zone, and plant measurements. The measurement period was from November, 2011 to November, 2013. Results from the study were documented in USGS Scientific Investigations Report 2017-5079.
The USGS Nevada Water Science Center recently completed a study to evaluate groundwater discharge by evapotranspiration (GWET) in sparsely vegetated portions of Amargosa Desert and improve understanding of hydrologic-continuum processes controlling groundwater discharge. Evapotranspiration and GWET rates were computed and characterized at three sites over 2 years using a combination of micrometeorological, unsaturated zone, and stable-isotope measurements. One site (AFS) was in a very sparse and isolated area of saltgrass (Distichlis spicata) where the depth to groundwater was 3.8 meters. The second site (AFD) was located in a sparse cover of predominantly shadscale (Atriplex confertifolia) where the depth to groundwater was 5.3 meters. The third site (ADRS), selected as a control site where GWET is assumed to be zero, was located in sparse vegetation dominated by creosote bush (Larrea tridentata) where the depth to groundwater was 110 meters.
Results indicated that capillary rise brought groundwater to within 0.9 meter (AFS) and 3 meters (AFD) of land surface, and that GWET rates were largely controlled by the slow but relatively persistent upward flow of water through the unsaturated zone in response to atmospheric-evaporative demands. Greater GWET at AFS (50 ± 20 millimeters per year (mm/yr)) than at AFD (16 ± 15 mm/yr) corresponded with its shallower depth to the capillary fringe and constantly higher soil-water content. The stable-isotope dataset for hydrogen (δ2H) and oxygen (δ18O) illustrated a broad range of plant-water-uptake scenarios. AFS saltgrass and AFD shadscale responded to changing environmental conditions and their opportunistic water use included the time- and depth-variable uptake of unsaturated-zone water derived from a combination of groundwater and precipitation. These results can be used to estimate GWET in other areas of Amargosa Desert where hydrologic conditions are similar.